The Index of Prohibited Books, or simply "Index", is used in a restricted sense to signify the exact list or catalogue of books, the reading of which was once forbidden to Catholics by the highest ecclesiastical authority. This list formed the second and larger part of the codex entitled "Index librorum prohibitorum", which contained the entire ecclesiastical legislation relating to books. (The "Index librorum prohibitorum", as an integrant part of the prohibition of books, has already been dealt with in the article .)

A book was prohibited or put on the Index by decree of the Sacred Congregation of the Roman Inquisition, of the Sacred Office, or of the Index, which decree though approved by the pope (in formâ communi), always remained a purely congregational decree. It need scarcely be mentioned that the pope alone, without having recourse to any of the congregations, could put a book on the Index, either by issuing a Bull or a Brief, or in any other way. Formerly it was the rule that a book was examined by one of the Roman Congregations only after complaint had been made to Rome. With regard to the Congregation of the Index, however, Pius X, when reorganizing the Roman Curia by the Constitution "Sapienti consilio" (29 June, 1908), decreed as follows:

Henceforth it will be the task of this Sacred Congregation not only to examine carefully the books denounced to it, to prohibit them if necessary, and to grant permission for reading forbidden books, but also to supervise, ex officio, books that are being published, and to pass sentence on such as deserve to be prohibited. Its further task is to remind the bishops of their sacred duty to combat the publication of pernicious writings and give information about them to the Apostolic See, in accordance with the Constitution Officiorum ad munerum of 25 January, 1897 (Acta S. Sedis, XLI, 432).

In the reorganization of the Roman Congregations, Pius X did not change the constitution or methods of the Congregation of the Index, but rather confirmed anew Leo XIII's Bull "Officiorum", together with Benedict XIV's "Sollicitae provida" sanctioned therein. This Bull of Benedict XIV, published on 8 July, 1753, regulated in detail the procedure of the Roman Congregations in the examination of pernicious books. It strictly commanded that the examination of a book be entrusted only to revisors well versed in the particular language and branch of learning. They were to be free from all partisanship and prejudice, and had to pass judgment not according to their private predilections or the tenets of any school but simply and solely according to the general Catholic teaching and the dogmas of Holy Church. Especially when examining books of Catholic authors of merit, they were to allow them free circulation, if at all possible, in a spirit of fairness and leniency. In no case was the book of a Catholic author to be condemned on the strength of the verdict of one revisor, not even when all the consultors agreed with him. Together with the report of the first revisor - who remained anonymous - the book was given to another for a second revision, and only when the second revisor's verdict was in agreement with that of the first were both reports referred to the cardinals for final decision. If, however, the second revisor thought that the book ought not to be prohibited, a third would examine both verdicts as well as the book itself, but without knowing the names of the other revisors. If the opinion of the third coincided with that of the first and with the general vote of the consultors, the case could be passed on to the cardinals. Otherwise the consultors were again to give their votes, whereupon the matter would be put before the cardinals for final decision.

In the case of writings which, according to the decision of the congregation, could be published in a revised edition, the congregation was to try, if possible, to hear the author's own defence or else appoint a consultor ex officio for the defence. If the book had been forbidden with the clause "donec corrigatur" (i.e. until corrected), and the author was willing to publish an edition in keeping with the wishes and orders of the congregation, the decree of prohibition was to be withheld, unless the prohibited edition was already widely circulated and known. In the latter case, when promulgating the decree, the new revised edition was to be expressly mentioned as authorized. The secretary to the Congregation of the Index was empowered to communicate the strictures passed on censured books to the respective authors or their representatives - but to these only at the author's request. Otherwise the official secret was to be strictly observed by all who had taken part in the process. Books, which at first sight were recognized as very dangerously heretical or immoral, could be forthwith prohibited.

The first printed catalogues of forbidden books did not appear at Rome, and, even after the institution of civil censorship, lists of books and writings prohibited by the State continued to appear. The first Roman "Index of Prohibited Books" (Index librorum prohibitorum), published in 1559 under Paul IV, was very severe, and was therefore mitigated under that pontiff by decree of the Holy Office of 14 June of the same year. It was only in 1909 that this "Moderatio Indicis librorum prohibitorum" (Mitigation of the Index of Prohibited Books) was rediscovered in "Codex Vaticanus lat. 3958, fol. 74", and was published for the first time in the "Zentralblatt für Bibliothekswesen" (Leipzig, 1909-10). Concerning the curious indexes of 1590 and 1593, which were printed but never promulgated see Hilgers, "Der Index der verbotenen Bücher", 12 sq., 524 sqq., 529 sqq.